1. Field of the Invention
The present invention relates to a sol-gel process for the preparation of aromatic polyimide aerogels, carbon aerogels, metal carbide aerogels which possess high surface area, uniform pore size, and narrow pore size distribution. The present invention also relates to aromatic polyimide aerogels, carbon aerogels, metal carbide aerogels of the invention having one or more metals dispersed therein. The present invention further provides processes for producing carbon aerogels and carbon aerogel derivatives from the polyimide aerogels of the invention such that the resultant carbon aerogels retain the interconnecting pore morphology of polyimide aerogel with high surface area, average pore size at 10 to 30 nm, and narrow pore size distribution.
2. Description of the Related Art
Aerogels are solid materials that consist of a highly porous network of micro-sized and meso-sized pores. The pores of an aerogel can frequently account for over 90% of the volume when the density of the aerogel about 0.05 gram/cc. Aerogels are generally prepared by a supercritical drying technique to remove the solvent from a gel (a solid network that encapsulates its solvent) such that no solvent evaporation can occur and consequently no contraction of the gel can be brought by capillary forces at its surface. Therefore, aerogel preparation through a sol-gel process proceeds in 3 steps: dissolution of the solute in a solvent, formation of the sol, formation of the gel, and solvent removal by either supercritical drying technique or any other method that removes solvent from the gel without causing pore collapse.
Typically, the synthesis of polyimide gels at very low solute concentration is the first step in the preparation of polyimide aerogels. Precursor poly(amic acids) are imidized in solution at elevated temperatures, some polyimides will gel as the reaction solution is quenched from the high reaction temperature to ambient temperature. However, solution imidization at elevated temperatures is accompanied by hydration leading to depolymerization of the poly(amic acids) and results in a weakened gel. Such gels do not have sufficient mechanical strength to yield low-density polyimide aerogels. It has been reported that chemical imidization of some poly(amic acids) at a solute concentration above 10–15% (wt./wt.) produces gels probably induced by intermolecular cross-linking. Such gels are mechanically weak and the high solute concentrations are not feasible for producing a low-density aerogel.
The commonly used organic precursors for carbon aerogels are resorcinol-formaldehyde (RF), polyacrylonitrile (PAN), and polyurethane. Although mesopores of carbon aerogels are very uniform, there are always a small percentage of micro-pores. For example, carbon aerogels prepared from RF aerogel are mesoporous materials with high surface areas. About 20 to 25% of micropores is formed during the pyrolysis of RF aerogel. When the carbon aerogel is used as catalyst support, the micropores impose a strong barrier for the mass transport of liquid components in or out of the pore. As a result, the catalysts in the micropores are severely underutilized.
Transition metal catalysts, such as platinum, nickel, cobalt, iron, and chromium, can easily be incorporated into the carbon aerogels by dissolving the corresponding soluble metallic compound precursors in the organic reaction solution before gelation occurs. The transition metal precursor compound is co-gelled with the organic gel or the precursor metal compound is precipitated onto the organic gel during or after the formation of the organic gel but before solvent removal, such that the molecular clusters of transition metal catalysts are uniformly distributed in the carbon matrix after pyrolysis of the organic aerogels.
Transition metal carbides are characterized by high melting points, hardness, and resistance to corrosion. Monolithic metallic carbides are traditionally prepared by hot pressing a metal carbide powder or hot pressing a powder mixture of carbon and a metal oxide compound under high pressure and temperatures above 1600° C. One method for preparing metal carbide aerogels in the form of a low density monolith comprises a uniform mixing of carbon and a metal at a molecular or colloidal level and pyrolyzing the mixture under conditions conducive to reaction of the metal and carbon to form a metal carbide during the pyrolysis process.
An efficient, inexpensive, and straightforward route to synthesize transition and main-group metal oxide aerogels have been reported by Alexander E. Gash, etc. in Journal of Non-Crystalline Solids 285 (2001), 22. In this approach, the epoxides are used as gelation agents for the metal oxide aerogel synthesis from simple metal ion salts. This methodology is modified in the present invention to produce interpenetrating network of metal oxide and polyimide aerogels.
Considerable effort has been devoted to the development and characterization of new electrode materials with improved performance for applications in energy storage devices such as electrochemical supercapacitors. Supercapacitors are unique devices exhibiting 20 to 200 times greater capacitance than conventional capacitors mainly due to the high surface area of the electrodes used or to highly functionalized surfaces. The large capacitance exhibited by these systems arises from double layer (DL) capacitance (i.e., from charge separation across the electrode/electrolyte interfacial DL) often in combination with pseudocapacitance. This pseudocapacitance is associated with redox-type reactions due to the presence of surface chemical groups and/or to participation of adsorbed species on its surface.
Carbon aerogels have been incorporated into electrodes in various electrochemical applications. U.S. Pat. No. 6,332,990 recites composite carbon thin film sheets which are used as electrodes in a variety of electrochemical energy storage applications wherein the carbon thin film sheet comprises a carbon aerogel as a binder. U.S. Pat. No. 5,358,802 teaches phosphoric acid doped carbon aerogels and the use of same as electrolytes in secondary lithium ion batteries. U.S. Pat. No. 5,601,938 recites membrane electrode assemblies for fuel cell application in which the gas diffusion layer comprises a carbon aerogel having transition metals and phosphoric acid deposited thereon. U.S. Pat. No. 6,544,648 recites new amorphous carbon materials which have been consolidated under elevated temperature and pressure and the use of such materials in electrochemical and structural applications.
U.S. Pat. No. 5,260,855, issued to Kaschmitter, recites a series of carbon foam electrodes which are prepared by pyrolysis of resorcinol-formaldehyde and related polymers. Kaschmitter teaches the use of such carbon foams as electrodes in capacitors.
It would be desirable to provide polyimide aerogels from aromatic dianhydrides and diamine monomers such as aromatic diamines or a mixture of at least one aromatic diamine monomer and at least one aliphatic diamine monomer such that the polyimide aerogel possesses low density, meso-pores, narrow pore size distribution and good mechanical strength. It would also be desirable to provide carbon aerogels having a surface area in excess of about 800 m2/gram or more preferably in excess of about 1000 m2/gram, while substantially or completely excluding micro-pores from the aerogel. It would further be desirable to provide carbon aerogels, carbon xerogel-aerogel hybrids, transition metal carbide aerogels and transition metal carbide-carbon hybrid aerogels possessing high surface areas, which may optionally be impregnated with highly dispersed transition metal clusters or particles.